6Accessing the LOINC Ontology from a FHIR Terminology Server
When working with the LOINC Ontology, leveraging a FHIR terminology server can simplify access and management. The server allows you to handle the content as a specific CodeSystem version, providing access to all FHIR Terminology operations.
LOINC Extension FHIR URIsing
The SNOMED International URI standard specifies that identify unversioned editions (i.e. editions) and versioned editions (i.e. versions) take the following respective forms:
These URIs apply to the 'version' element of codes and the 'URL' parameter in FHIR Operations. The 'system" element value is always 'http://snomed.info/sct' for all SNOMED editions. For more information on the URI Specification, please refer to the URI Standard document.
Examples
The following table shows some examples of URIs for editions and versions relevant when working with the LOINC Ontology.
Resource
URI
SNOMED CT International Edition
SNOMED CT International Edition, 20250101
SNOMED CT-LOINC
SNOMED CT-LOINC, 31 March 2025
FHIR Operations
The following table lists a few key requests relevant for retrieving content from the LOINC Ontology using the FHIR Terminology services API.
Service Name and Status
Input
Output
$lookup
A conceptId and a CodeSystem url
Concept metadata, including status, designations, etc.
$expand
Get all members of the reference set
REQUIRED
A reference set specified by its refsetId, in this case the identifiers for the set of Orderable and Observable LOINC Terms:
LOINC Orderable Reference Set
$validate-code
Test if a concept or description is a member of a specified reference set
REQUIRED
A reference set specified by its refsetId
A candidate concept.id
If the candidate concept is a member of the reference set: TRUE
Otherwise: FALSE
Please refer to the SNOMED CT Terminology Services Guide for a comprehensive guidance on SNOMED CT enabled terminology services.
Example FHIR Requests
These examples use the concept 635111010000100 |LOINC Orderable Reference Set| as the refsetId.
Service Name
API Call
Result
Get concept metadata
GET fhir]/ValueSet/$expand
?url=http://snomed.info/sct?fhir_vs=refset/[refsetId]
&code=537131010000109Returns a JSON representation of concept metadata
Get all members of the reference set
GET [fhir]/ValueSet/$expand
?url=http://snomed.info/sct?fhir_vs=refset/[refsetId]
&count=10for example
GET [fhir]/ValueSet/$expand
?url=http%3A%2F%2Fsnomed.info%2Fsct%3Ffhir_vs%3Drefset%2F635111010000100&count=10An alternative solution is to use the expression constraint language, as shown here:
GET [fhir]/ValueSet/$expand
?url=http%3A%2F%2Fsnomed.info%2Fsct%3Ffhir_vs%3Decl%2F%5E%5B635111010000100%5D
&count=10GET [fhir]/ValueSet/$expand
?url=http%3A%2F%2Fsnomed.info%2Fsct%3Ffhir_vs%3Decl%2F%5E635111010000100&count=10Returns a JSON representation of data about each of the reference set member.
The data returned for each concept includes:
code: the conceptId of the reference set member
display: the preferred term for the reference set member
Also returns the total number of reference set members
As some reference sets have very large numbers of children, this service is paged. Requests parameters include:
count to restrict the number of members returned.
offset to specify the start in the results (in multiples of the limit).
Test if a concept is a member of the reference set
GET [fhir]/ValueSet/$validate-code
?system=http://snomed.info/sct&code=537131010000109 &url=http://snomed.info/sct/[moduleId]/version/[effectiveTime]?fhir_vs=refset/[refsetId]for example
GET [fhir]/ValueSet/$validate-code
?system=http://snomed.info/sct&code=537131010000109 &url=http://snomed.info/sct/900000000000207008/version/20200131?fhir_vs=refset/635111010000100Returns a JSON object with a "Parameters" resource, including a "result" parameter with a boolean value (true/false) indicating whether the member validated OK or not against the reference set.
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